BME 312 BIOMEDICAL INSTRUMENTATION II LECTURER: ALİ IŞIN
LECTURE NOTE 6
Implantable Defibrillators
FACULTY OF ENGINEERING
DEPARTMENT OF BIOMEDICAL ENGINEERING
The implantable cardioverter defibrillator (ICD)
• ICD is a therapeutic device that can detect ventricular tachycardia or fibrillation and automatically deliver high-voltage (750 V)
shocks that will restore normal sinus rhythm.
• Advanced versions also provide low-voltage (5–10 V) pacing stimuli for painless
termination of ventricular tachycardia and for
Figure 1: An Implantable Cardioverter Defibrillator and a Pacemaker
• The implantable defibrillator has evolved
significantly since first appearing in 1980. The newest devices can be implanted in the
patient’s pectoral region and use electrodes
that can be inserted transvenously, eliminating the traumatic thoracotomy required for
placement of the earlier epicardial electrode
systems.
• Transvenous systems provide rapid, minimally invasive
implants with high assurance of success and greater patient comfort.
• Advanced arrhythmia detection algorithms offer a high
degree of sensitivity with reasonable specificity, and extensive monitoring is provided to document performance and to
facilitate appropriate programming of arrhythmia detection and therapy parameters.
• Generator longevity can now exceed 5 years, and the cost of providing this therapy is declining.
Pulse Generators
• The implantable defibrillator consists of a
primary battery, high-voltage capacitor bank,
and sensing and control circuitry housed in a
hermetically sealed titanium case.
• Implantable defibrillator circuitry must include;
• an amplifier, to allow detection of the millivolt-range cardiac electrogram signals,
• noninvasively programmable processing and control functions, to evaluate the sensed cardiac activity and to direct generation and delivery of the therapeutic energy,
• high-voltage switching capability,
• dc-dc conversion functions to step up the low battery voltages,
• random access memories, to store appropriate patient and device data,
• radiofrequency telemetry systems, to allow communication to and from the implanted device.
• Defibrillators must convert battery voltages of approximately 6.5 V to the 600–750 V needed to defibrillate the heart.
• Since the conversion process cannot directly supply this high voltage at current strengths needed for defibrillation, charge is accumulated in
relatively large ( ≈ 85–120 μ F effective capacitance) aluminum electrolytic capacitors that account for 20–30% of the volume of a typical defibrillator.
• These capacitors must be charged periodically to prevent their dielectric from deteriorating. If this is not done, the capacitors become electrically leaky, yielding excessively long charge times and delay of therapy.
• Power sources used in defibrillators must have sufficient capacity to
provide 50–400 full energy charges (≈34 J) and 3 to 5 years of bradycardia pacing and background circuit operation.
• They must have a very low internal resistance in order to supply the
relatively high currents needed to charge the defibrillation capacitors in 5–
15 s. This generally requires that the batteries have large surface area electrodes and use chemistries that exhibit higher rates of internal discharge than those seen with the lithium iodide batteries used in pacemakers.
• The most commonly used defibrillator battery chemistry is lithium silver vanadium oxide
.
Electrode Systems (“Leads”)
• Early implantable defibrillators utilized patch electrodes
(typically a titanium mesh electrode) placed on the surface of the heart, requiring entry through the chest
• This procedure is associated with approximately 3–4%
perioperative mortality, significant hospitalization time and complications, patient discomfort, and high costs. Although subcostal, subxiphoid, and thoracoscopic techniques can minimize the surgical procedure, the ultimate solution has been development of fully transvenous lead systems with acceptable defibrillation thresholds.
Figure 2: Early Epicardial ICD design with Patch Electrodes
• Currently available transvenous leads are
constructed much like pacemaker leads, using polyurethane or silicone insulation and
platinum-iridium electrode materials.
• These lead systems use a combination of two or
more electrodes located in the right ventricular apex, the superior vena cava, the coronary sinus, and
sometimes, a subcutaneous patch electrode is placed in the chest region.
• These leads offer advantages beyond the avoidance of major surgery. They are easier to remove should there be infections or a need for lead system
revision.
• Lead systems are being refined to simplify the implant procedures. One approach is the use of a single catheter having a single right ventricular low-voltage electrode for pacing and detection, and a pair of high-voltage defibrillation electrodes spaced for replacement in the right ventricle and in the superior vena cava (Figure 3 a). A more recent approach parallels that used for unipolar pacemakers. A single right-ventricular catheter having bipolar pace/sense electrodes and one right ventricular high-
voltage electrode is used in conjunction with a defibrillator housing that serves as the second high-voltage electrode (Figure 3 b). Mean biphasic pulse defibrillation thresholds with the generator-electrode placed in the patient’s left pectoral region are reported to be 9.8 ± 6.6 J ( n = 102). This approach appears to be practicable only with generators suitable for
• Figure 3 a. The latest transvenous fibrillation systems employ a single catheter placed in the right ventricular apex. In panel a, a single transvenous catheter provides defibrillation electrodes in the superior vena cava and in the right ventricle. This catheter provides a single pace/sense electrode which is used in conjunction with the right ventricular high-voltage defibrillation electrode for arrhythmia detection and antibradycardia/antitachycardia pacing (configuration that is sometimes referred to as integrated bipolar ).
• Figure 3 b. With pulse generators small enough to be placed in the pectoral region, defibrillation can be achieved by delivering energy between the generator housing and one high-voltage electrode in the right ventricle (analogous to unipolar pacing) as is shown in panel b. This catheter provided bipolar pace/sense electrodes for arrhythmia detection and
Arrhythmia Detection
• Most defibrillator detection algorithms rely primarily on heart rate to indicate the
presence of a treatable rhythm. Additional refinements sometimes include simple
morphology assessments, as with the
probability density function, and analysis of
rhythm stability and rate of change in rate.
• The probability density function evaluates the percentage of time that the filtered ventricular electrogram spends in a window centered on the baseline
• The rate-of-change-in-rate or onset evaluation discriminates sinus tachycardia from ventricular tachycardia on the basis of the typically gradual acceleration of sinus rhythms versus the relatively abrupt acceleration of many pathologic tachycardias.
• The rate stability function is designed to bar detection of tachyarrhythmias as long as the variation in ventricular rate exceeds a physician-programmed tolerance, thereby reducing the likelihood of inappropriate therapy delivery in response to atrial fibrillation.
• Because these additions to the detection algorithm reduce sensitivity, some defibrillator designs offer a supplementary detection mode that will trigger therapy in response to any elevated ventricular rate of prolonged duration.
• These extended-high-rate algorithms bypass all or portions of the normal detection screening, resulting in low specificity for rhythms with prolonged elevated rates such as exercise-induced sinus tachycardia. Consequently, use of such algorithms generally increases the incidence of inappropriate therapies.
Arrhythmia Therapy
• Pioneering implantable defibrillators were capable only of defibrillation shocks.
Subsequently, synchronized cardioversion
capability was added. Antibradycardia pacing had to be provided by implantation of a
standard pacemaker in addition to the
defibrillator, and, if antitachycardia pacing was prescribed, it was necessary to use an
antitachycardia pacemaker
• But currently marketed implantable defibrillators offer integrated ventricular demand pacemaker function and tiered antiarrhythmia therapy
(pacing/cardioversion/defibrillation).
• Availability of devices with antitachy pacing capability significantly increases the acceptability of the
implantable defibrillator for patients with ventricular tachycardia.
• Human clinical trials have shown that biphasic defibrillation waveforms are more effective than monophasic waveforms, and newer devices now incorporate this characteristic.
Speculative explanations for biphasic superiority include the large voltage change at the transition from the first to the second phase or hyperpolarization of tissue and reactivation of sodium channels during the initial phase, with resultant tissue conditioning that allows the second phase to more readily excite the myocardium.
Implantable Monitoring
• Previously, defibrillator data recording capabilities were quite limited, making it
difficult to verify the adequacy of arrhythmia detection and therapy settings.
• The latest devices record electrograms and diagnostic channel data showing device
behavior during multiple tachyarrhythmia
• These devices also include counters (number of events detected, success and failure of each programmed therapy, and so on) that present a broad, though less specific, overview of
device behavior (Figure 4)
• Electrogram storage has proven useful for documenting false therapy delivery due to atrial fibrillation, lead fractures, and sinus tachycardia, determining the triggers of arrhythmias; documenting rhythm
accelerations in response to therapies; and
demonstrating appropriate device behavior
when treating asymptomatic rhythms.
• Electrograms provide useful information by themselves, yet they cannot indicate how the deviceinterpreted cardiac activity.
• Increasingly, electrogram records are being
supplemented with event markers that indicate how the device is responding on a beat-by-beat basis.
These records can include measurements of the sensed and paced intervals, indication as to the
specific detection zone an event falls in,indication of
Follow-up
• Defibrillator patients and their devices require careful follow-up.
• After implantation these complications may occur ;
• infection requiring device removal,
• Postoperative respiratory complications,
• postoperative bleeding and/or thrombosis,
• lead system migration or disruption,
• documented inappropriate therapy delivery, most commonly due to atrial fibrillation,
• transient nerve injury,
• asymptomatic subclavian vein occlusion,
• pericardial effusion ,
• subcutaneous patch pocket hematoma,
• Pulse generator pocket infection,
• lead fracture,
• lead system dislodgement.
• Although routine follow-up can be accomplished in the clinic, detection and analysis of transient events depends on the recording capabilities available in the devices or on the use of various external monitoring
Conclusion
• The implantable defibrillator is now an established and
powerful therapeutic tool. The transition to pectoral implants with biphasic waveforms and efficient yet simple transvenous and subcutaneous lead systems is simplifying the implant
procedure. These advances are making the implantable
defibrillator easier to use, less costly, and more acceptable to patients and their physicians
